1. Field of the Invention
An embodiment of the disclosed invention relates to a semiconductor device including a transistor or including a circuit having such a transistor. For example, an embodiment of the present invention relates to a semiconductor device including a transistor in which a channel formation region is formed using an oxide semiconductor or including a circuit having such a transistor.
2. Description of the Related Art
A technique for manufacturing a transistor or the like by using an oxide semiconductor film for a channel formation region and applying it to a display device has been attracting attention. Examples of such a transistor include a transistor in which zinc oxide (ZnO) is used as an oxide semiconductor film and a transistor in which InGaO3(ZnO)m is used as an oxide semiconductor film. A technique for manufacturing such a transistor including an oxide semiconductor film over a light-transmitting substrate and applying it to a switching element or the like of an image display device is disclosed in Patent Documents 1 and 2.
An inverted staggered (bottom-gate) transistor in which an oxide semiconductor film containing indium (In), gallium (Ga), and zinc (Zn) is used as a semiconductor layer and a buffer layer using a metal oxide are provided between the semiconductor layer and source and drain electrode layers is disclosed in Patent Document 3. In this transistor, an ohmic contact is formed by intentionally providing the metal oxide layer as the buffer layer between the semiconductor layer and the source and drain electrode layers.